In one known method, the opening of the passive pressure limiting valve is monitored. An opened pressure limiting valve is detected following a load reduction if the rail pressure exceeds a limit, a steady-state operating state of the combustion engine is subsequently detected again and if a characteristic of the closed-loop control system deviates significantly from a reference value. Characteristic of the closed-loop control system is understood as the I component of the rail pressure controller or, for example, a PWM signal for acting on the suction throttle. A functional rail pressure sensor is imperative for the method presented.
Another known approach, is directed to a method for monitoring a passive pressure limiting valve following a load reduction. In a first step, a check is made whether the rail pressure, proceeding from a steady-state rail pressure, for example, 1800 bar, has exceeded a first, higher limit, for example 1850 bar. A check is then made in a second step whether the rail pressure exceeds a second, even higher limit, for example 1920 bar, in spite of a temporary charging of the suction throttle in the closing direction. If both limits have been exceeded, the pressure limiting valve is set as open. However, due to the variation of pressure limiting valves, it may be the case in practice that although the pressure limiting valve is detected as open by the evaluation program, said valve is actually still closed. This results in an operator error alarm and an erroneous follow-up response. A functional rail pressure sensor is also imperative for this method.